Soft magnetic materials are used for various applications, such as core materials in inductors, stators and rotors for electrical machines, actuators, sensors and transformer cores. Traditionally, soft magnetic cores, such as rotors and stators in electric machines, are made of stacked steel laminates. Soft magnetic composites may be based on soft magnetic particles, usually iron-based, with an electrically insulating coating on each particle. By compacting the insulated particles optionally together with lubricants and/or binders using the traditionally powder metallurgy process, soft magnetic components may be obtained. By using the powder metallurgical technique it is possible to produce such components with a higher degree of freedom in the design, than by using the steel laminates as the components can carry a three dimensional magnetic flux and as three dimensional shapes can be obtained by the compaction process.
The present invention relates to an iron-based soft magnetic composite powder, the core particles thereof being coated with a carefully selected coating rendering the material properties suitable for production of inductors through compaction of the powder followed by a heat treating process.
An inductor or reactor is a passive electrical component that can store energy in form of a magnetic field created by the electric current passing through said component. An inductors ability to store energy, inductance (L) is measured in henries (H). Typically an inductor is an insulated wire winded as a coil. An electric current flowing through the turns of the coil will create a magnetic field around the coil, the filed strength being proportional to the current and the turns/length unit of the coil. A varying current will create a varying magnetic field which will induce a voltage opposing the change of current that created it.
The electromagnetic force (EMF) which opposes the change in current is measured in volts(V) and is related to the inductance according to the formula;v(t)32 Ldi(t)/dt
(L is inductance, t is time, v(t) is the time-varying voltage across the inductor and i(t) is the time-varying current.)
That is; an inductor having an inductance of 1 henry produces an EMF of 1 volt when the current through the inductor changes with 1 ampere/second.
Ferromagnetic- or iron-core inductors use a magnetic core made of a ferromagnetic or ferrimagnetic material such as iron or ferrite to increase the inductance of a coil by several thousand by increasing the magnetic field, due to the higher permeability of the core material.
The magnetic permeability, μ, of a material is an indication of its ability to carry a magnetic flux or its ability to become magnetised. Permeability is defined as the ratio of the induced magnetic flux, denoted B and measured in newton/ampere*meter or in volt*second/meter2, to the magnetising force or filed intensity, denoted H and measured in amperes/meter, A/m. Hence magnetic permeability has the dimension volt*second/ampere*meter. Normally magnetic permeability is expressed as the relative permeability μr=μ/μ0, relative to the permeability of the free space, μ0=4*η*10−7 Vs/Am. Permeability may also be expressed as the inductance per unit length, henries/meter.
Magnetic permeability does not only depend on material carrying the magnetic flux but also on the applied electric field and the frequency thereof. In technical systems it is often referred to the maximum relative permeability which is maximum relative permeability measured during one cycle of the varying electrical field.
An inductor core may be used in power electronic systems for filtering unwanted signals such as various harmonics. In order to function efficiently an inductor core for such application shall have a low maximum relative permeability which implies that the relative permeability will have a more linear characteristic relative to the applied electric filed, i.e. stable incremental permeability, μΔ (as defined according to ΔB=μΔ*ΔH), and high saturation flux density. This enables the inductor to work more efficiently in a wider range of electric current, this may also be expressed as that the inductor has “good DC-bias”. DC-bias may be expressed in terms of percentage of maximum incremental permeability at a specified applied electrical field, e.g. at 4 000 A/m. Further low maximum relative permeability and stable incremental permeability combined with high saturation flux density enables the inductor to carry a higher electrical current which is inter alia beneficial when size is a limiting factor, a smaller inductor can thus be used.
One important parameter in order to improve the performance of soft magnetic component is to reduce its core loss characteristics. When a magnetic material is exposed to a varying field, energy losses occur due to both hysteresis losses and eddy current losses. The hysteresis loss is proportional to the frequency of the alternating magnetic fields, whereas the eddy current loss is proportional to the square of the frequency. Thus at high frequencies the eddy current loss matters mostly and it is especially required to reduce the eddy current loss and still maintaining a low level of hysterisis losses. This implies that it is desired to increase the resistivity of magnetic cores.
In the search for ways of improving the resistivity different methods have been used and proposed. One method is based on providing electrically insulating coatings or films on the powder particles before these particles are subjected to compaction. Thus there are a large number of patent publications which teach different types of electrically insulating coatings. Examples of published patents concerning inorganic coatings are the U.S. Pat. Nos. 6,309,748, 6,348,265 and 6,562,458. Coatings of organic materials are known from e.g. the U.S. Pat. No. 5,595,609. Coatings comprising both inorganic and organic material are known from e.g. the U.S. Pat. Nos. 6,372,348 and 5,063,011 and the DE patent publication 3,439,397, according to which publication the particles are surrounded by an iron phosphate layer and a thermoplastic material. European Patent EP1246209B1 describes a ferromagnetic metal based powder wherein the surface of the metal-based powder is coated with a coating consisting of silicone resin and fine particles of clay minerals having layered structure such as bentonite or talc.
U.S. Pat. No. 6,756,118B2 reveals a soft magnetic powder metal composite comprising a least two oxides encapsulating powdered metal particles, the at least two oxides forming at least one common phase.
The patent application JP2002170707A describes an alloyed iron particle coated with a phosphorous containing layer, the alloying elements may be silicon, nickel or aluminium. In a second step the coated powder is mixed with a water solution of sodium silicate followed by drying. Dust cores are produced by moulding the powder and heat treat the moulded part in a temperature of 500-1000° C.
Sodium silicate is mentioned in JP51-089198 as a binding agent for iron powder particles when producing dust cores by moulding of iron powder followed by heat treating of the moulded part.
In order to obtain high performance soft magnetic composite components it must, also be possible to subject the electrically insulated powder to compression moulding at high pressures as it is often desired to obtain parts having high density. High densities normally improve the magnetic properties. Specifically high densities are needed in order to keep the hysterisis losses at a low level and to obtain high saturation flux density. Additionally the electrical insulation must withstand the compaction pressures needed without being damaged when the compacted part is ejected from the die. This in turn means that the ejection forces must not be too high.
Furthermore, in order to reduce the hysterisis losses, stress releasing heat treatment of the compacted part is required. In order to obtain an effective stress release the heat treatment should preferably be performed at a temperature above 300° C. and below a temperature, where the insulating coating will be damaged, about 700° C., in an atmosphere of for example nitrogen, argon or air.
The present invention has been done in view of the need for powder cores which are primarily intended for use at higher frequencies, i.e. frequencies above 2 kHz and particularly between 5 and 100 kHz, where higher resistivity and lower core losses are essential. Preferably the saturation flux density shall be high enough for core downsizing. Additionally it should be possible to produce the cores without having to compact the metal powder using die wall lubrication and/or elevated temperatures. Preferably these steps should be eliminated.
In contrast to many used and proposed methods, in which low core losses, are desired, it is an especial advantage of the present invention that it is not necessary to use any organic binding agent in the powder composition, which powder composition is later compacted in the compaction step. The heat treatment of the green compact can therefore be performed at higher temperature without the risk that the organic binding agent decomposes; a higher heat treatment temperature will also improve the flux density and decrease core losses. The absence of organic material in the final, heat treated core also allows that the core can be used in environments having elevated temperatures without risking decreased strength due to softening and decomposition of an organic binder and improved temperature stability is achieved.